The present invention relates generally to methods of polishing semiconductor wafers, more particularly to chemical/mechanical polishing of silicon semiconductor wafers.
Silicon semiconductor wafers are the building blocks of integrated circuits. Silicon wafers are produced from crystal ingots which are grown and sliced into individual silicon wafers. The wafers are then subjected to grinding to planarize their front and back surfaces and etching to remove damage created by the slicing and grinding. At least one surface of the wafer is then subjected to chemical/mechanical polishing (CMP) before they are cleaned and inspected for packaging. The purpose of the CMP process in silicon wafer manufacturing is to remove surface damage caused during crystal slicing and chemical and mechanical shaping of the wafer and to achieve a highly reflective surface having the required flatness or planarity, which is of critical importance in the manufacture of high-density integrated circuits.
In general, CMP processing involves holding and pressing semiconductor wafers against a polishing pad mounted to a rotating turntable in the presence of a polishing solution. The polishing is achieved through a combination of pressure, temperature, mechanical abrasive force and chemical reaction. The polishing solution typically comprises an abrasive slurry and an alkaline etchant. During polishing, the abrasive slurry such as colloidal silica (SiO2) or alumina (Al2O3) slurry mechanically abrades the semiconductor wafer surfaces while the alkaline etchant chemically reacts with the surface of the wafer to further remove damage and to achieve better surface quality.
Silicon wafers are typically polished using a CMP method including two or more polishing steps. The first step, or rough polishing, uses a relatively coarse slurry, aggressive chemistry, and a harder polishing pad for fast stock removal. Subsequent polishing steps or finish polishing uses finer slurry, reduced wafer-to-pad contact pressure, relatively less aggressive chemistry and a softer polishing pad for better surface quality. Conventionally, the wafer is rinsed at least once with deionized water between polishing steps to wash away residual chemicals and prevent the abrasive slurry particles from being carried over to the subsequent polishing step.
One of the major concerns with CMP methods is achieving sufficient reduction of the surface defect level on polished wafers. Defects during CMP may be caused by carryover of coarse slurry particles from one polishing step to the next, continued chemical reactions on the wafer surface, which may lead to etching stains due to the incomplete removal of polishing solution, and contamination from airborne chemicals. Defective wafers have to be rejected and therefore cause yield losses in the CMP process.
Several strategies have been designed to reduce surface defects formed during CMP processing. One example is quenching the polishing solution as disclosed by Krishna et al. in U.S. Pat. No. 5,571,373. In quenching, the surface of the wafer is contacted with an acidic quench solution at the end of the polishing step while the wafer is still engaged on the polishing pad. The quench solution acts to neutralize the polishing solution on the wafer surface. A surfactant may be included in the quench solution to help wet the wafer surface and make the wafer surface hydrophilic. A hydrophilic wafer surface allows for more efficient removal of residual slurry, thus providing the wafer with a higher resistance to the attachment of foreign objects on the wafer surface as has been suggested in PCT/US98/08936 and by Huynh et al., U.S. Pat. No. 5,704,987.
Another strategy as taught by Prigge et al., U.S. Pat. No. 4,973,563, is the formation of a hydrophobic protective film on the wafer surface by contacting the wafer with a surfactant such as alkyl silane or short chain alkyl alcohols after the application of the polishing slurry. When silane is used, the first molecule layer chemically adsorbs at the wafer surface as reported by Fadeev et al., Langmuir, 1999, vol. 15, p. 3759. A chemically adsorbed layer is difficult to remove and risks causing high organic contamination on the wafer surface. Thus, an additional oxidation step may be required to remove the chemically adsorbed organic film as described in U.S. Pat. No. 4,973,563. Likewise, alcohol is not preferred because a high concentration is required to form the hydrophobic protective film. This high consumption of reagents is costly and presents environmental issues.
Therefore, a need exists for a simple method of reducing surface defects, including etching stains, on polished semiconductor wafers.
Among the several objects of this invention, therefore, may be noted the provision of an improved semiconductor wafer polishing process; the provision of such a polishing process which improves the yield of a semiconductor production run; and the provision of such a polishing process wherein the occurrence of etching stains on the polished surface of the semiconductor wafers is substantially reduced and the wafer surfaces are substantially protected from airborne contaminants.
Briefly therefore, the present invention is directed to a method for polishing semiconductor wafers. The method comprises applying a polishing solution to a polishing pad and contacting the polishing pad and the polishing solution with a surface of a semiconductor wafer as the wafer moves relative to the polishing pad to polish the surface of the wafer. A surfactant is applied to the polished surface of the wafer and the wafer is disengaged from the polishing pad. After application of the surfactant to the polished surface of the wafer, the polished surface of the disengaged wafer is dried for a period of at least about 20 seconds prior to contacting the polished surface of the wafer with a subsequent process liquid such as a rinse media (e.g., deionized water) or a polishing solution in a subsequent polishing step.
The present invention is further directed to a process for polishing a semiconductor wafer including at least two polishing steps. The method comprises applying a first polishing solution to a first polishing pad and contacting the first polishing pad and the first polishing solution with a surface of a semiconductor wafer as the wafer moves relative to the polishing pad to polish the surface of the wafer. An acidic quench solution is applied to the wafer to neutralize the first polishing solution and a water-soluble, polyoxyethylene glycol surfactant is applied to the polished surface of the wafer. The wafer is disengaged from the polishing pad. After application of the surfactant to the polished surface of the wafer, the polished surface of the disengaged wafer is dried for a period of at least about 20 seconds prior to contacting the polished surface of the wafer with a rinse media. A second polishing solution is then applied to a second polishing pad and the rinsed, polished surface of the wafer is contacted with the second polishing pad and the second polishing solution as the wafer moves relative to the polishing pad to further polish the surface of the wafer.
The present invention is still further directed to a method of polishing a semiconductor wafer wherein a substantially continuous hydrophobic protective layer is formed across the polished surface of the wafer. The method comprises applying a polishing solution to a polishing pad and contacting the polishing pad and the polishing solution with a surface of the wafer as the wafer moves relative to the polishing pad to polish the surface of the wafer. A water-soluble polyoxyethylene glycol surfactant is applied to the polished surface of the wafer. The wafer is disengaged from the polishing pad and a substantially continuous hydrophobic layer comprising the polyoxyethylene glycol surfactant is formed across the polished surface of the disengaged wafer.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.